A few initial thoughts, from reading your post:
1) Average utilisation over (say) a day can often be misleading. It's no good your infrastructure carrying next to nothing between 4PM and 8AM, if it's totally swamped for the rest of the time - but if the stats you look at are only averaged over the day, you very likely won't see the problem. You need to look at more granular stats during the periods that people are having the issue.
2) 90 - 100% utilisation of an MX for any length of time is likely to cause problems for many of the applications flowing through that MX. Maybe look at waht traffic is required to run through the MX - asssuming your inter-VLAN traffic is all L3 routed on the MX, what are the major traffic contributors, application and client-wise? Could they, for instance, be located in the same VLAN, without overly damaging your security?
3) In regard to 3, I would certainly look at what your cameras & NVR etc. are communicating with, and how that would be handled by your LAN setup
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